At the present time a number of different proposals are under consideration for extending the amount of information transmitted in a composite television signal. Various such proposals include the transmission of stereophonic sound; bilingual sound transmission accompanying a single video presentation; continuously transmitted time and temperature information or other digital information which can be decoded and displayed either as text on the screen or supplied to a separate display device; and the like. The composite television signal is a complex signal already carrying large amounts of information, including the picture or video information, sound information, color subcarrier information used to operate color television receivers, horizontal and vertical synchronizing information, blanking, etc. Consequently, to add additional information to the already complex signal generally results in compromises which degrade some other portion of the signal which already is present and necessary.
Various systems have been proposed in the past to add additional auxiliary signals to the composite television signal with a corresponding modification of television receivers to permit them to utilize the auxiliary signals. A number of systems, particularly, have been designed in the past to add an additional sound channel; so that stereophonic or bilingual sound capabilities may be obtained.
Generally these systems place the added or auxiliary signal somewhere in the horizontal blanking interval. For example, two such systems are disclosed in the patents to Hodge, U.S. Pat. No. 3,446,914, and to Rout, U.S. Pat. No. 3,466,387. In both of these patents the auxiliary sound information is placed on the "back porch" of the blanking interval. A location on the back porch for audio or other auxiliary information appears to be a logical choice. This location, however, is used for the color subcarrier signal of the standard composite color television signal. If additional auxiliary information is to be included in this location of the composite signal, it is necessary to extend the length of the back porch to adequately separate the color subcarrier from the added auxiliary signal. Alternatively, it would be necessary to shorten the number of cycles of the color subcarrier. Even if sufficient room is present for both the color and auxiliary signals on the back porch of the blanking interval, this back porch does not carry through the vertical synchronization pulse interval; so that if auxiliary sound information is being transmitted at this location in the composite signal, there are recurring interruptions at the vertical rate. Since this rate is at a 60 Hz frequency, these interruptions may carry over into the sound processing amplifiers and be reproduced by the loudspeakers. This, obviously, is highly undesirable.
Another system where the auxiliary signal is placed on the back porch of the video signal is the system disclosed in the Justice U.S. Pat. No. 3,902,007. The system of this patent uses an extended time on the back porch of alternate lines. This takes time from the video portion of the composite signal (because the back porch has been extended), so that there is some degredation in the video signal content available to be reproduced in the picture at the receiver. In addition, because of this location, the system is not capable of producing 15 KHz audio due to the inherent limitations of the location. Justice, like the systems of Rout and Hodge is also subject to the transmission interruptions of the auxiliary signal during the vertical intervals of the composite television signal. Consequently, this interruption also would result in a disturbance in the audio channels if the auxiliary signal is utilized is a bilingual or other audio signal.
Another approach which has been taken is to place auxiliary sound or other information directly on the synchronization pulse by means of a notch in the synchronization pulse. This location is not subject to the disadvantage of the 60 Hz interruption present when the auxiliary signal is placed on the back porch of the signal; but by notching the synchronization pulses, the energy content of such pulses is reduced in direct proportion to the width of the notch provided for the auxiliary signal transmission. A system of this type is disclosed in the patent to Felgel-Farnholz, U.S. Pat. No. 2,268,001. Since there is a notch in the synchronization pulse of such systems, the receiver is made more suseptible to noise, resulting in a degredation of its performance characteristics.
A different approach which has been attempted for adding auxiliary signals is disclosed in the patent to Campiani, U.S. Pat. No. 4,134,127. In Campiani every second burst of the color subcarrier is replaced with a new burst of a different frequency carrying the auxiliary information. Obviously, a substantial risk of degrading the color synchronization performance of the receivers is present with this approach and, again, it is not capable of reproducing 15 KHz audio signals.
The patent to Fujio, et al., U.S. Pat. No. 3,723,637, discloses a system superimposing digital information on the color burst by means of superimposed amplitude modulated digital signal information having the frequency and phase of the color burst signal. This results in a relatively complex system implementation, both at the transmitter and the receiver and, in addition, is subject to degredation of the color subcarrier signal. The Fujio et al system, however, apparently is capable of transmitting audio information having a 15 KHz frequency as contrasted with other systems which are discussed above and which do not have this capability.
Various other systems have been developed in an attempt to place a modulated auxiliary signal of a multiple-phase modulated carrier on the synchronization pulses. This position, however, is not desirable since it is inherently capable of causing overmodulation of the transmitter. If an attempt is made to ensure that overmodulation does not occur, the modulation of the auxiliary signal is reduced to a point where its effectiveness is impaired, or clipping of the auxiliary signal exists. In any event, a location on the synchronization pulses themselves has been found to be undesirable.
Accordingly, it is desirable to provide a location for, and a receiver responsive to, auxiliary signals not subject to the disadvantages noted above.